1. Field of the Invention
This invention relates generally to precision measuring machines such as coordinate measuring machines and, more specifically, to apparatuses and methods for calibrating measuring probes on such machines.
2. Related Art
Precision measuring machines, such as coordinate measuring machines, are commonly used for dimensional inspection of workpieces such as machine parts. Typically, a workpiece to be measured is secured to a fixed table, and a measuring probe is secured to a component of the machine, which is movable along three coordinate axes. To measure the position of a point on a workpiece, the probe is brought into proximity with the point and the x, y and z coordinates of the probe position are recorded.
Typical configurations of coordinate measuring machines include bridge-type machines and horizontal arm machines. In bridge-type machines, a bridge is supported by, and movable along, y-axis guideways; the bridge supports a carriage which is movable in the x-axis direction; the carriage supports a vertical component of the machine, commonly referred to as a ram, which is movable in the z-axis. The probe assembly is attached to the lower end of the ram. In a horizontal arm machine, a horizontal arm to which a probe assembly is attached is supported by a z-axis carriage movable on a z-axis rail; the z-axis rail is deflected from vertical depending on both the position of the carriage and the distance by which the arm is extended. Some state of the art coordinate measuring machines have refinements such as rotatable probe assemblies, which are rotatable about two independent axes of rotation. Other refinements include the use of capacitance and laser probes, for example.
The accuracy of conventional coordinate measuring machines is limited by inaccuracies in the calibration of its measuring probe. Over time, the probe calibration degrades due to a well known phenomenon commonly referred to as calibration drift, which may be caused by many mechanical or environmental factors. For example, heat generation can cause rapid calibration drift and subsequent degradation in measurement accuracy in certain probes such as capacitance and laser probes. One approach to improve the accuracy in such machines is to design probe assemblies which minimize heat generation, heat build-up, or dimensional instability caused by heat effects. This approach, however, can be technically challenging and expensive to implement.
Alternatively, probes can be more frequently calibrated to minimize the effects of calibration drift on measurement accuracy. Current technology in common usage for probe calibration utilizes a calibration object for both the initial calibration and for periodic recalibration. In typical arrangements, the calibration object is attached to the worktable at a fixed location in the measurement volume. Calibration drift is detected by returning the probe to the fixed location of the calibration object and making measurements of coordinate positions on the calibration object.
Many disadvantages with these conventional calibration strategies exist. For example, one disadvantage is the lost time required to move the probe to the fixed location at which the calibration object is located. This lost time results in an increase in the overall time required to measure objects. In addition, because the calibration object is fixed in the measurement volume, the effective area available for the workpiece is reduced. In some instances, the calibration object is removed from the measurement volume. Thus, an additional drawback with a conventional calibration strategy is the time associated with the periodic reinstallation of the calibration object.